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Drought tolerant forb flora of a semi-arid protected savanna in the Lowveld of South Africa

Authors:
  • South African Environmental Observation Network-Ndlovu Node

Abstract

Background: Increased frequency and intensity of droughts related to climate change are predicted to induce pressure on herbaceous communities. Considering that forbs contribute significantly to savanna ecosystem resilience, we investigated forb communities of a protected semi-arid savanna during an extensive drought.Objective: We identified drought-tolerant species with their related functional traits.Results: Drought-tolerant forb flora comprised of several plant families and species with overlapping traits, of which the ability to resprout was related to perennials, whereas succulence and prostrate growth form were typical annual forb dominance traits.Conclusion: Results highlight the functional importance of forbs and their resilience to drought events in protected areas.
BOTHALIA – African Biodiversity & Conservation
ISSN: (Online) 2311-9284, (Print) 0006-8241
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Authors
H. van Coller1
J. Klem1
F. Siebert1
Aliations
1Unit for Environmental Sciences
and Management, North-West
University, Private Bag X6001,
Potchefstroom 2520, South
Africa.
Corresponding Author
Dr H. van Coller
helga.vancoller@nwu.ac.za.
Dates
Submitted: 20 September 2019
Accepted: 2 July 2020
Published: 24 February 2021
How to cite this article:
Van Coller, H., Klem, J. & Siebert, F.,
2021, ‘Drought tolerant forb ora
of a semi-arid protected savanna
in the Lowveld of South Africa’,
Bothalia 51(1), a10. http://dx.doi.
org/10.38201/btha.abc.v51.
i1.10
Background: Increased frequency and intensity of droughts related to climate
change are predicted to induce pressure on herbaceous communities. Consider-
ing that forbs contribute significantly to savanna ecosystem resilience, we inves-
tigated forb communities of a protected semi-arid savanna during an extensive
drought.
Objective: We identified drought-tolerant species with their related functional
traits.
Results: Drought-tolerant forb flora comprised of several plant families and spe-
cies with overlapping traits, of which the ability to resprout was related to peren-
nials, whereas succulence and prostrate growth form were typical annual forb
dominance traits.
Conclusion: Results highlight the functional importance of forbs and their resil-
ience to drought events in protected areas.
Keywords: resprouting; herbaceous communities; functional traits; resilience;
climate change.
Introduction
Predicted increasing drought intensity and frequency, combined with higher
average temperatures due to global climate change, are threatening biodiver-
sity, and therefore the stability, functioning and sustainability of terrestrial eco-
systems (Barros et al. 2018). Drought is a common phenomenon in semi-arid
rangelands (Vetter 2009) and has been shown to cause rapid and lasting effects
on vegetation dynamics and ultimately ecosystem function and services (Barros
et al. 2018; Ploughe et al. 2019).
Depending on intensity, droughts can cause shifts in plant species assemblages,
leading to the establishment of different plant communities (Junk et al. 2018).
In herbaceous layers of semi-arid savannas, these communities are composed
mainly of annual grasses and both annual and perennial forbs (O’Connor
1998; Buitenwerf et al. 2011). In the savanna context, the term ‘forb’ is used
to classify anything other than trees, shrubs and grasses, which has led to a
poor functional definition of this life form. For the purpose of this study, we
will define forbs as non-graminoid vascular plants with limited woody tissue
and with perennating buds at or below soil surface. Drought episodes tend
to favour forbs, since they possess a variety of drought-tolerant traits such as
underground storage organs (Siebert et al. 2019) associated with persistent
bud banks and viable seed banks (Siebert & Dreber 2019). Despite being as-
sociated with savanna land degradation and therefore perceived as being an
undesirable functional group by land managers (Fynn & O’Connor 2000; Tes-
sema et al. 2011), forbs are important through providing ecosystem functions
Drought tolerant forb ora of a semi-arid
protected savanna in the Lowveld of South Africa
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(i.e. forage stability) during stressed conditions, and
functional redundancy to absorb disturbances such as
sustained grazing and droughts (Van Coller et al. 2018).
Forbs are an important source of nutritious forage and
may constitute an important part of ungulate diets at
certain times of the year (Du Toit 2003; Van Der Merwe
& Marshal 2012). Moreover, forbs contribute significant-
ly to the biodiversity of savanna and grassland systems
(Buitenwerf et al. 2011; Siebert & Scogings 2015), which
are functionally diverse, suggesting a stronger resilience
to different environmental conditions (Turner & Knapp
1996; Van Coller et al. 2018). Forb ecology research in
dry African savannas generally report on forb responses
at the level of functional group rather than species level.
Furthermore, variations in forb functional traits defining
plant strategies for regeneration and survival in adapta-
tion to climate extremes such as droughts, remain un-
derstudied (Siebert & Dreber 2019).
Below-average rainfall in the Central Lowveld of South
Africa was recorded for two consecutive years (2015
and 2016) (Swemmer et al. 2018). Using data collected
during this time, we sought to identify specific drought
tolerant forb species and their respective functional
traits in a semi-arid African savanna. In doing so, we
aimed to enhance knowledge of the attributes that al-
low these forb species to persist during droughts and
potential functions that they fulfil under such environ-
mental conditions.
Materials and Methods
Forb communities were studied in the semi-arid savan-
na of the greater Kruger National Park (KNP). Protect-
ed areas are not exempt from natural disasters, such as
drought. They therefore provide valuable natural exper-
imental settings where spatial heterogeneity and eco-
logical responses function under natural drivers (Pickett
et al. 2003). These areas host a variety of indigenous
wildlife including mixed feeders (e.g. elephants [Lox-
odonta africana (Blumenbach, 1797)]; impala [Aepy-
ceros melampus (Lichtenstein, 1812)]), browsers (e.g.
greater kudu [Tragelaphus strepsiceros (Pallas, 1766)];
bushbuck [Tragelaphus sylvaticus (Sparrman, 1780)]),
and grazers (e.g. blue wildebeest [Connochaetes tau-
rinus (Burchell, 1823)]; plains zebra [Equus quagga
(Boddaert, 1785)], amongst others (Van der Waal et al.
2011; Scogings et al. 2012).
Field surveys were undertaken at two sites of similar
geology (i.e. granite and gneiss), but different soil nu-
trient statuses (i.e. nutrient-rich sodic soil versus nutri-
ent-poor sandy soil). Floristic and functional trait data
were collected from 48 plots of 1 m2 (18 plots within the
nutrient-rich site and 30 plots within the nutrient-poor
site) during the usual rainy season (November–March)
of the extensive drought of 2015/2016. In the KNP (i.e.
nutrient-rich site), total annual rainfall was 200 mm be-
low the mean annual rainfall for the area (Van Coller et
al. 2018), while at the nutrient-poor site in Timbavati
Private Nature Reserve (TPNR) it was ~330 mm below
the long-term average (Kaschula et al. 2005). Within
each plot, forbs were identified up to species level and
all individuals counted. Frequency per species was cal-
culated with respect to all recorded species in each re-
spective site. Frequency is considered a stable variable
for the abundance of an individual species (O’Connor
2015). Frequency measures (%) were used to identi-
fy forb species most commonly observed in the study
sites. Only forb species with a frequency 1 are dis-
cussed. Functional traits were assigned to forb species
(Cornelissen et al. 2003; Germishuizen & Meyer 2003)
based on the potential contribution to the functioning
of semi-arid protected areas (i.e. palatability, life history
and nitrogen-fixing ability), as well as the ability to toler-
ate conditions related to drought and herbivory (growth
form, life history, resprouting capacity and succulence).
Results and Discussion
Herbaceous productivity is strongly affected by rainfall,
and generally reveals marked deterioration in response
to drought conditions (Figure 1) (Fynn & O’Connor
2000; O’Connor 2015). Despite this, frequency mea-
sures revealed a total of 31 forb species among the two
study sites. A mean number of six and four forb species
was recorded per plot (1 m2) in the nutrient-rich and nu-
trient-poor sites respectively. The number of forb spe-
cies per plot recorded in the nutrient-rich site ranged
from one to 17, while a lower range (0–7) of forb spe-
cies were recorded for plots in the nutrient-poor site.
Plant families that comprised most of the frequent
taxa during the drought included Acanthaceae and
Figure 1. Sparsely vegetated sampling sites within the KNP (A)
and TPNR (B) during the drought.
A
B
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Amaranthaceae in the nutrient-rich site, and Faba-
ceae and Boraginaceae in the nutrient-poor site (Table
1). Prevalence of the Fabaceae is in accordance with
Wagner et al. (2016) who reported that nitrogen-fixing
herbaceous legumes from the Fabaceae may increase in
abundance after disturbances in dry savanna rangelands,
and also in disturbed grasslands (Muller et al. 2021). Ni-
trogen-fixing ability is a trait generally associated with
ecosystems with low nutrient availability (Cornelissen
et al. 2003), explaining the high frequency of Chamae-
crista mimosoides (L.) Greene in the nutrient-poor site
(Table 1). Over 50% of the most frequent forbs were
annuals (Table 1). Annual forbs have been reported to
form a major component of soil seed banks, especially
under heavy grazing (O’Connor 1991; Tessema et al.
2016), potentially enabling them to respond and estab-
lish rapidly when conditions become favourable (e.g.
smaller rainfall events interrupting extensive droughts).
The occurrence of bare soil caused by herbivores and
drought is known to facilitate the colonisation of pros-
trate forb species (Burkepile et al. 2016). Blepharis inte-
grifolia (L.f.) E.Mey. ex Schinz, a palatable, perennial and
low-growing forb is known to form patches of continu-
ous groundcover or ‘browsing lawns’ in heavily utilised
sodic bottomlands (Siebert & Scogings 2015), whilst the
prostrate-growing annual Gisekia africana (Lour.) Kuntze
revealed the highest frequency in the nutrient-poor site
(Table 1). Persistence of these species is therefore likely
attributed to the positioning of perennating tissue at or
close to the soil surface, since fewer species could have
buds far above the soil surface during harsher climat-
ic conditions such as drought (Cornelissen et al. 2003).
Moreover, erect growing plant species with their peren-
nating buds situated above the soil surface are especial-
ly susceptible to trampling, heavy grazing and exposure
to extreme heat conditions, whereas prostrate-growing
species are avoidant by retaining buds and leaf material
close to the soil surface (Cornelissen et al. 2003). A pros-
trate growth form in forbs could therefore be considered
an important resistance trait against drought and grazing
in the protected Lowveld savannas of South Africa.
The presence of annual forb species with a pioneer
character in seed banks (Tessema et al. 2016) allows
for their initial colonisation of bare soil (Siebert & Dre-
ber 2019). Therefore, some of the most frequently
observed forb species during the drought (i.e. Portu-
laca kermesina N.E.Br. and P. hereroensis Schinz in the
nutrient-rich site, and G. africana in the nutrient-poor
site) were annuals (Table 1). Moreover, these species
exhibited traits generally associated with grazing- and
drought-tolerance (i.e. prostrate growth form and suc-
culence) (Cornelissen et al. 2003). Although little is
known about succulence as a drought-tolerant trait in
forbs, the ability of these species to retain water in their
leaves and stems during dry conditions, together with a
prostrate growth form to avoid and tolerate herbivory,
possibly favoured their survival when subjected to her-
bivore utilisation in dry, hot conditions. Annual forbs
exhibiting this combination of traits therefore make up
an important component of the forb flora of semi-arid
protected areas, especially during a drought.
The majority of drought-tolerant perennial forbs in this
study had the ability to resprout, either through buds
located at or near the soil surface, or belowground.
Such a disturbance-tolerant trait is well-known for trees
and shrubs, but our understanding of regeneration from
buds in forbs is unknown and requires further investiga-
tion (Siebert & Dreber 2019).
Over half of the frequent forb species recorded during
the drought were palatable (Table 1). This reinforces the
functional importance of the forb component through
their ability to provide important ecosystem functions,
such as forage stability during stressed conditions, and
functional redundancy enabling them to absorb distur-
bances such as sustained grazing and drought (Van Col-
ler et al. 2018).
Conclusion
Despite anticipated deterioration of the herbaceous lay-
er during droughts, forbs have the ability to withstand
such disturbances through species-specific adaptions.
Numerous forb species were able to persist amidst the
abnormal hot and dry conditions, while providing the
ecosystem with important functions and services, such
as forage stability. Plant strategies for survival and re-
generation during drought conditions are species- and
family specific, which may vary across ecosystem types.
Furthermore, drought-adaptations were also specific
for life history groups, as annuals displayed strategies to
survive after emergence (e.g. succulence and prostrate
growth form), whilst the majority of perennial forbs had
the ability to resprout from a persistent bud bank. As
protected areas aim to conserve biodiversity, provide
forage security for wildlife and to maintain ecosystem
resilience, this study demonstrates that forbs contribute
to these at plant taxonomic and functional trait levels.
Authors’ contributions
HvC (North-West University) collected vegetation data
from the Nkuhlu exclosures at the Kruger National
Park (KNP), analyzed and reported data and wrote the
manuscript, whilst JK (North-West University) collected
data from Timbavati Private Nature Reserve (TPNR).
FS (North-West University) was the project leader, pro-
moter and supervisor to HvC and JK respectively, and
was responsible for project design, data collection, re-
porting and the writing of the manuscript.
Disclaimer
The authors declare that the work presented, and views
expressed in this submitted article is their own and is
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Table 1: Frequent forb species in the semi-arid savanna of the Greater KNP during a drought with a summary of relevant functional traits. Trait data were derived from field guides and various Floras
Species Family Life history Growth form Palatability§Resprouting capacity Nitrogen fixator Succulence Frequency
Nutrient-rich site (KNP)
Blepharis integrifolia (L.f.) E.Mey. ex Schinz Acanthaceae Perennial Prostrate High Aboveground No No 45.8
Portulaca kermesina N.E.Br. Portulacaceae Annual Prostrate High No Leaves and stems 5.8
Ruellia cordata Thunb. Acanthaceae Perennial Erect leafy High Belowground No No 4.4
Phyllanthus incurvus Thunb. Euphorbiaceae Perennial Erect leafy Moderate Aboveground No No 4.0
Bidens bipinnata L.*Asteraceae Annual Erect leafy Moderate No No 2.8
Waltheria indica L.* Sterculiaceae Perennial Erect leafy Low Aboveground No No 2.6
Portulaca hereroensis Schinz Portulacaceae Annual Prostrate Low No Leaves and stems 2.5
Acalypha indica L. Euphorbiaceae Annual Erect leafy Moderate No No 2.1
Commelina benghalensis L. Commelinaceae Annual Erect leafy High Above and belowground No No 2.1
Hibiscus micranthus L.f. Malvaceae Perennial Erect leafy High Belowground No No 2.1
Kyphocarpa angustifolia (Moq.) Lopr. Amaranthaceae Annual Erect leafy High No No 2.1
Ocimum americanum L. Lamiaceae Perennial Erect leafy Low Aboveground No No 2.1
Evolvulus alsinoides (L.) L. Convolvulaceae Annual Erect leafy Moderate No No 1.4
Gomphrena celosiodes Mart.*Amaranthaceae Perennial Prostrate Moderate Aboveground No No 1.4
Justicia flava (Vahl) Vahl Acanthaceae Perennial Erect leafy Moderate No No 1.4
Achyranthes aspera L.* Amaranthaceae Perennial Erect leafy High No No 1.2
Indigofera filipes Benth. ex Harv. Fabaceae Annual Erect leafy Moderate Yes No 1.2
Nutrient-poor site (TPNR)
Gisekia africana (Lour.) Kuntze Gisekiaceae Annual Prostrate Low No Leaves and stems 35.3
Chamaecrista mimosoides (L.) Greene Fabaceae Annual Erect leafy Moderate Yes No 11.2
Heliotropium strigosum Willd. Boraginaceae Annual Erect leafy Low No No 6.0
Commelina benghalensis L. Commelinaceae Annual Erect leafy High Above and belowground No No 4.8
Leucas sexdentata Skan Lamiaceae Annual Erect leafy Low No No 4.4
Phyllanthus maderaspatensis L. Euphorbiaceae Perennial Erect leafy Moderate Aboveground No No 4.1
Chamaecrista absus (L.)Irwin & Barneby Fabaceae Annual Erect leafy Low Ye s No 2.3
Phyllanthus parvulus Sond. Euphorbiaceae Perennial Erect leafy Moderate No No 2.3
Schkuhria pinnata (Lam.) Cabrera* Asteraceae Annual Erect leafy Low No No 1.8
Sida ovata Forsk. Malvaceae Annual Erect leafy Low Belowground No No 1.8
Bidens bipinnata L.*Asteraceae Annual Erect leafy Moderate No No 1.6
Heliotropium ovalifolium Forssk. Boraginaceae Annual Erect leafy Low Belowground No No 1.6
Heliotropium ciliatum Kaplan Boraginaceae Perennial Erect leafy Low Belowground No No 1.4
Tephrosia longipes Meisn. Fabaceae Annual Erect leafy Low Aboveground No No 1.4
§ This categorical trait is based on an adapted version of Walker’s Palatability Index (Siebert & Scogings 2015), to detect browsing signs from smaller ungulates. Species preference of larger herbivores
(e.g. elephant) remains poorly known as they often uproot the entire plant. For the purpose of this trait data, a forb was considered palatable if it showed any signs of browsing, irrespective of the
type of large herbivore; Alien species are indicated with asterisk (*).
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not an official position of the institution or funder. The
authors furthermore declare that they have no financial
or personal relationships that may have inappropriately
influenced them in writing this article.
Source(s) of support
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‘Herbaceous legume encroachment reduces grass produc-
tivity and density in arid rangelands’, PLoS ONE, 11(11),
1–13. https://doi.org/10.1371/journal.pone.0166743.
... Besides fire and herbivory, plants in semi-arid savannas also have to deal with drought events. A diverse suite of belowground bud-bearing organs and a persistent bud bank can also provide insurance against such events and some studies indeed reported the rapidly release of perennial forb species after drought events (O'connor 1995;Siebert and Dreber 2019;Siebert et al. 2020;Smit and Bond 2020;Van Coller et al. 2020). Although we did not test for specific drought responses, our results indicated the drought event between 2015 and 2016 did not overweight the effects of fire on the belowground bud bank, reinforcing that disturbance exclusion was the main depreciative fact for the bud bank and belowground organs community in our study. ...
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